1. Field
The present disclosure relates to a dibenzylated polybenzimidazole based polymer and a method for preparing the same.
2. Description of the Related Art
Fuel cells are energy conversion systems by which chemical energy of fuel is converted directly into electrical energy, and are next-generation power generating systems that cause substantially no contamination while providing high efficiency. Among various fuel cells, proton exchange membrane fuel cells (also referred to as PEMFC hereinafter) are driven at low temperature and provide high efficiency, and thus are used in various fields, including power sources for transportation, portable power sources and main power sources for houses. However, PEMFC uses a noble metal Pt catalyst and expensive perfluorosulfonated polymer electrolyte such as Nafion. As a result, PEMFC has a significant limitation in its commercialization. Therefore, there is an imminent need for a novel type of fuel cell.
To overcome the above problem of PEMFC, active studies have been conducted about alkaline fuel cells (also referred to as AFC hereinafter). AFC uses a low-cost hydroxyl (OH−) ion exchange electrolyte and non-noble metal catalyst. Particularly, such a non-noble metal catalyst shows high activity in alkaline atmosphere, unlike PEMFC. As a result, AFC can overcome the disadvantages of PEMFC with ease.
Recently, many studies have been conducted to develop a solid electrolyte that substitutes for a liquid electrolyte used in the conventional AFC. When using a solid electrolyte capable of exchange of hydroxide ions, since a solid electrolyte membrane has high ion conductivity with which hydroxide ions can be transported in a neutral state, the AFC using a solid electrolyte membrane is advantageous in that it avoids a need for circulation of strong alkaline solution, unlike the conventional AFC. In addition, a solid electrolyte membrane allows a fuel cell to be driven in a neutral state, thereby solving the problem of corrosion caused by the use of strong alkali. Further, in the case of a solid alkaline exchange membrane fuel cell (also referred to as SAEMFC hereinafter), no precipitation of potassium carbonate (K2CO3) occurs in the membrane. By virtue of the above-mentioned advantages, many attentions have been given to development of solid alkaline exchange membrane fuel cells (SAEMFC).
To transport hydroxide ions, specific functional groups are required. Particular examples of such functional groups include quaternary ammonium groups. Polymers, such as poly(ether)sulfone (PES), poly(ether ether ketone) (PEEK) and polystyrene, can transport hydroxide ions when they adopt quaternary ammonium groups. In addition to quaternary ammonium groups, other functional groups such as guadinium, phosphonium and imidazolium may be used. However, because most of such polymers have low quality and poor long-term stability, it is required to develop a novel electrolyte membrane having high quality and long-term stability.
Polymer electrolyte membranes capable of transporting alkali ions such as hydroxide ions (OH−), carbonate ions (CO32−) and potassium carbonate ions (KCO3−) have been produced by using polybenzimidazole derivatives. It has been found that although polybenzimidazole itself has low ability of transporting the corresponding ions, two methyl groups bound to the two nitrogen positions of polybenzimidazole can improve the ability significantly.
However, according to the studies conducted by the present inventors, when dimethylated polybenzimidazole is subjected to ion exchange in a strong alkaline solution such as potassium hydroxide (KOH) solution, the polymer undesirably undergoes decomposition of a benzimidazole ring due to the attack of strong hydroxide ions.